Nearly nine years ago, the Institute of Electrical and Electronics Engineers (IEEE) assigned a task group to the development of the 802.15.4 standard. The wireless communications industry was growing beyond home and office use to include environments such as industrial, medical, and more. These new applications increasingly depended on embedded systems, giving rise to the need for a network protocol with a natively low-power physical layer.
To make the IEEE 802.15.4 standard low power and low cost, the task group took the following approaches:
- Reduce the amount of data transmitted
- Reduce the transceiver duty cycle and the frequency of data transmissions
- Reduce the frame overhead—the number of noninformation bytes need to send a given number of information bytes—to improve communication efficiency and to minimize power usage.
- Reduce complexity
- Reduce transmission range
- Implement strict power management mechanisms through power-down and sleep modes
Figure 1. A diagram of the 802.15.4 architecture. Protocols such as ISA 100.11a and ZigBee are implemented in the upper layers
More specifically, certain decisions were made within IEEE 802.15.4's architectural layers (Figure 1) to assist in power and cost savings. For power savings, one method is to maximize the data rate for any given amount of data that needs to be transmitted—IEEE 802.15.4 allows data rates up to 250 Kbps. The high data rate allows the standard to accommodate applications that require higher data throughput, but that also can tolerate the increased latency to decrease power consumption. For cost savings, several bandwidth options are included in the physical layer (PHY). The 2.4 GHz frequency is unlicensed and widely available for use; low-cost designs for this band exist, making device manufacturing more affordable. The 868 MHz and 915 MHz bands are freely available in Europe and the United States, respectively.
Key Features of IEEE 802.15.4
- Data rates of 250 Kbps, 40 Kbps, and 20 Kbps
- Two addressing modes: 16-bit short and 64-bit IEEE addressing
- Support for critical-latency devices, such as joysticks
- Carrier-sense multiple-access with collision avoidance (CSMA-CA)
- Automatic network establishment by the coordinator
- Fully handshaked protocol for transfer reliability
- Power management to ensure low power consumption
- 16 channels in the 2.4 GHz ISM band, 10 channels in the 915 MHz ISM band, and one channel in the 868 MHz band
How Does IEEE 802.15.4 Compare to Other Standards?
802.11 is a wireless local area network (WLAN) while 802.15.4, Bluetooth, and Wireless USB are wireless personal area networks (WPANs). The big differences are range, bandwidth, and architecture. A WLAN can cover a wide area—an entire home, business, or factory floor—while a WPAN is a much smaller wireless network with a range of a few yards. In addition, WLANs usually have a larger bandwidth, which means that they can carry a lot more data than a WPAN. On the architecture side, a WLAN is a true network that can support a variety of network topologies such as mesh and hub and spoke. In contrast, a WPAN is usually more of a point-to-point network, connecting one device at a time to another device. 802.15.4 can be reclassified as a WLAN because it has been used in products and standards such as ISA 100.11a to support applications with wider ranges and different network topologies.
Power consumption is based on transmission range and data rate. If the application does not require a lot of data to be sent over a large distance, then it is possible to develop a low-power WLAN or WPAN. Low-power 802.15.4 networks are often used for industrial applications that require long range, high reliability, and minimal data to be transferred. In contrast, 802.11 networks, which require a lot more power, are useful for long-range networks that are required to transfer a very large amount of data. It is the data transfer and the required bandwidth that drives up the energy consumption for these two similar networks.
There are a variety of uses within industrial applications for which 802.15.4 is ideally suited. Here we list three of the most common.
Machinery automation. Many heavy industrial and automated assembly lines rely on machines that hold, turn, twist, cut, and move materials. Many of these machines also have sensors attached to their moving components to monitor and help control the assembly lines' performance. Unfortunately, wires and cable assemblies do not endure for long when they are twisted and turned. In some factories, some wires need to be repaired every three or four months. In addition to the expense of troubleshooting and then performing repairs, the assembly line is shut down during that time. In contrast, battery-operated wireless sensors and switches are rugged and do not have issues with twisting and turning, so they can handle the nonstop motion required on an assembly line.
Cable replacement. Another common application for 802.15.4 wireless is to replace cables and wires that are often impacted by exposure to corrosive chemicals, grease, dust, paint, heat, and metal fragments. Factories can be dirty places, rife with hazardous materials. In some assembly lines, the products themselves must be dipped, sprayed, or coated with corrosive chemicals that can destroy the insulation around wires. Once again, a battery-operated sensor or switch, with suitably rugged packaging, can withstand this type of abuse and can function safely in the most inhospitable factory environment.
Flexible installation. Maybe the most important reason for the rapid adoption of industrial wireless is its flexibility. On many assembly lines, buttons and switches are used to control various functions as well as emergency responses. The ability to locate a wireless switch or button close to the operators, rather than adjacent to power and communication cables, can save many minutes of walking every day.
In addition, many factories and assembly lines experience semi-regular reconfigurations to accommodate new-product production or to fine tune existing lines to make the same product more efficiently. The ability to move a wireless switch or controller as needed makes reconfiguring the entire system much easier and quicker. The facility manager can mount the controller or switch where it is most effective, without having to worry about rerouting power and communication cables. In addition, whenever a new assembly line goes into operation, there is always a period when the line manager realizes that the controller or switch is poorly located and should be installed somewhere else. Once again, wireless makes that type of fine-tuning relatively painless.
Building on Basic 802.15.4
The key benefits of IEEE 802.15.4—and the reasons why sensor developers keep this standard at the top of their lists—include its low power consumption and reliability (made possible by the physical (PHY) and MAC layer protocols) as well as its affordability. Alone, it meets many application needs for today's industrial environments. However, it is possible to improve upon the basic reliability, security, and robustness features as set out by 802.15.4.
On the reliability front, Honeywell engineers were able to increase end-to-end data reliability (Figure 2) by using communication algorithms. If there is strong interference (i.e., a metallic obstruction in the communication path) or if the receiver is off, the device will continue to attempt to transmit the message, sleeping in between to save battery life, until it is successful.
Figure 2. Honeywell's Limitless wireless solutions use a variety of antennas, chosen based on the application performance needs, to ensure the communication is within range and to help alleviate interference
By using built-in unique keys that are specific to each customer system, it is possible to improve security to thwart hacking, eavesdropping, and cross-communication with other devices. Key generation is random to avoid duplication. As applications vary in their sensitivities and level of security, the flexibility IEEE 802.15.4 offers in security architecture can have a big influence on a sensor developer. AES 128-bit-based cryptography is used to povide data confidentiality, data authenticity, and replay protection.
When it comes to improving robustness, it's really about battery life—a major benefit of the IEEE 802.15.4 standard. With an ultra-low power draw, battery life can last from months to years. There are many factors that contribute to the life of a battery, including sensor reporting intervals, sensor type, battery type, and ambient temperatures. Honeywell has boosted battery life by building in device sleep times between transmissions as well as adding an application layer to further enable a proven long-life product.
From the standard architecture, to the flexibility of adding in features unique to a specific customer's use, IEEE 802.15.4 can be an ideal wireless solution for countless industrial applications. It offers choices, low cost, low power consumption, and reliable and secure performance.
ABOUT THE AUTHORS
Joseph Citrano III is Global Product Manager and Ramakrishna Budampati is Engineering Manager for Honeywell Sensing and Control, Golden Valley, MN. Joseph Citrano can be reached at 763-954-4654, [email protected]; Ramakrishna Budampati can be reached at 763-954-6332, [email protected].